XmR Individuals Chart Formulas
The XmR (Individuals and Moving Range) chart can help you evaluate a process when there is only one measurement and they are farther apart: monthly postage expense and so on.
Here are the formulas used to calculate both the Moving Range and Individuals chart in QI Macros.
Moving Range Chart formula
Individuals X chart formula
k = number of data points
for n=2 observations (moving range is calculated from two samples, the present minus the previous value)
D4 = 3.267
D3 = 0
E2 = 2.660
Conforms with ANSI/ASQC B1, B2, B3 1996
QI Macros XmR Charts employ the Nelson Rule for Moving Range
"Any moving range (Ri) greater than 3.5*Rbar should be removed from the average moving range calculation."
In example below, point 20 (46.50) is greater than 3.5*11.14 (original Rbar), so it will be removed from Rbar Calculation which changes Rbar to 9.73 and UCL/LCL for both the X and R charts.
Source: Control Charts for Individual Measurements, Lloyd S. Nelson, Journal of Quality Technology, Vol. 14, No. 34, July 1982.
Process: Calculate, plot, and evaluate the range chart first. If it is "out of control," so is the process. If the range chart looks okay, then calculate, plot, and evaluate the X chart.
- Create a XmR Individuals chart in Excel using QI Macros
- XmR Individuals chart video
- XmR Individuals chart template in Excel
The XmR Individuals chart is just one of the tools included in the QI Macros SPC Software for Excel.
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